The present invention relates in general to vapor coating and in particular to a new and useful method and apparatus of evaporating and evaporative substance under vacuum.
Known are electron beam evaporators operated with a high electron current (for example 100A) and a relatively low acceleration voltage (for example 100 V). Either hollow cathodes heating up through ion bombardment, or heated hot cathodes are employed for this purpose. Discharges based on electron sources of this kind will be termed low voltage arcs in the following. In this connection, a continuous supply of inert gas (such as argon) to the cathode is advantageous and makes it possible to produce well focused beams of high intensity with a low acceleration voltage. The gas serves the purpose of compensating for the space charge, so that a plasma is produced. Low voltage arcs can be concentrated and directed to the evaporative substance through magnetic fields, and have the great advantage of strongly activating the gas or residual vapor in the evaporation chamber. Prior art arrangements for evaporation by means of a low voltage arc have the disadvantage, however, that they can use only evaporative materials which are electrically conducting or become electrically conducting at the evaporation temperature. But they are frequently difficult to use even for refractory metals, since the achievable power density is not satisfactory. In addition, with an increasing evaporation rate, the vapor density above the evaporative substance continues to rise, so that the low energy electron beam of the low voltage arc loses the greatest part of its energy already in the vapor, and the energy transferred to the evaporative substance is insufficient. This leads to a critical limitation of the evaporation rate obtainable in practice.